51 research outputs found
Jeans instability of interstellar gas clouds in the background of weakly interacting massive particles
Criterion of the Jeans instability of interstellar gas clouds which are
gravitationally coupled with weakly interacting massive particles is revisited.
It is established that presence of the dark matter always reduces the Jeans
length, and in turn, Jeans mass of the interstellar gas clouds. Astrophysical
implications of this effect are discussed.Comment: version accepted in ApJ, Nov. 1, 1998 issue, vol. 50
Phenomenological model of propagation of the elastic waves in a fluid-saturated porous solid with non-zero boundary slip velocity
Zhu & Granick [Phys. Rev. Lett. 87, 096105 (2001)] have recently
experimentally established existence of a boundary slip in a Newtonian liquid.
They reported typical values of the slip length of the order of few
micro-meters. In this light, the effect of introduction of the boundary slip
into the theory of propagation of elastic waves in a fluid-saturated porous
medium formulated by Biot is investigated. The new model should allow to fit
the experimental seismic data in circumstances when Biot's theory fails, as the
introduction of phenomenological dependence of the slip velocity upon
frequency, which is based on robust physical arguments, adds an additional
degree of freedom to the model. If fact, it predicts higher than the Biot's
theory values of attenuation coefficients of the both rotational and
dilatational waves in the intermediate frequency domain, which is in
qualitative agreement with the experimental data. Therefore, the introduction
of the boundary slip yields three-fold benefits: (A) Better agreement of theory
with an experimental data since the parametric space of the model is larger
(includes effects of boundary slip); (B) Possibility to identify types of
porous medium and physical situations where boundary slip is important; (C)
Constrain model parameters that are related to the boundary slip.Comment: numerical error corrected; J. Acoust. Soc. Am. (accepted
Mixing of shear Alfven wave packets
The propagation of shear Alfven wave packets in inhomogeneous magnetic
fields, at the origin of their distortion, regardless of the occurrence of
non-linear coupling, is considered. It is shown that the distortion mechanism
can be regarded as mixing process and hence, standard "phase mixing"
corresponds to the effect of an "Alfvenic" shear flow while enhanced
dissipation at a magnetic X-point corresponds to mixing by an "Alfvenic" strain
flow. The evolution of the global wave field is supposed to result from the
dynamics of a superposition of wave packets and a kinetic equation for the wave
energy is obtained following this eikonal (WKB) description. Since shear Alfven
wave packets experience continuous shearing/straining while transported by an
inhomogeneous Alfvenic flow , their mixing process, in physical
space, is also a cascade of wave energy in k-space. The wave energy spectrum
resulting from this linear mechanism of energy transfer is determined for the
special case of waves propagating along chaotic magnetic field lines, the
analog of a chaotic mixing process. The latter follows a power-law, in
the energy conserving range in space.Comment: revised, final version, Astron. Astrophys. in pres
Missing bits of the solar jigsaw puzzle: small-scale, kinetic effects in coronal studies
The solar corona, anomalously hot outer atmosphere of the Sun, is
traditionally described by magnetohydrodynamic, fluid-like approach. Here we
review some recent developments when, instead, a full kinetic description is
used. It is shown that some of the main unsolved problems of solar physics,
such as coronal heating and solar flare particle acceleration can be viewed in
a new light when the small-scale, kinetic plasma description methods are used.Comment: 10 pages, 6 figure
On the conical refraction of hydromagnetic waves in plasma with anisotropic thermal pressure
A phenomenon analogous to the conical refraction widely known in the
crystalooptics and crystaloacoustics is discovered for the
magnetohydrodynamical waves in the collisionless plasma with anisotropic
thermal pressure. Angle of the conical refraction is calculated for the medium
under study which is predicted to be . Possible
experimental corroborating of the discovered phenomenon is discussed.Comment: 6 pages, REVTeX, Accepted in Physics of Plasma
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